The present invention generally relates to recording apparatus and more particularly, to an automatic document feeder (ADF) for use in a copying apparatus, a facsimile telegraph, a microfilm printer, etc., which is provided with an automatic sheet winding device. The automatic sheet winding device includes a rotary drum having a sheet retaining claw for retaining an original sheet, and a sheet clamping means for clamping the original sheet wound around a circumferential face of the rotary drum so as to automatically wind the original sheet around the circumferential face of the rotary drum such that the original sheet is subjected to exposure through the sheet clamping means by an exposure device, with the sheet retaining claw being provided on the circunferential face of the rotary drum.
Conventionally, in a high-speed copying apparatus, there is provided an automatic document feeder (ADF) in which after the contents of an original sheet wound automatically around a rotary drum have been read through exposure of the original sheet during rotation of the rotary drum, the original sheet is ejected from the rotary drum. The known ADF includes an automatic sheet winding device. In the known automatic sheet winding device, although it is most desirable that the original sheet and the rotary drum are brought into close contact with each other, such a phenomenon inevitably takes place that the original sheet deviates from the surface of the rotary drum due to stiffness of the original sheet.
Thus, the assignee assigned by the present inventors proposed in Japanese Patent Application No. 185123/1984, an automatic sheet winding device in which an original sheet is clamped onto a circumferential face of a rotary drum by using a sheet clamp as shown in FIG. 1. The sheet clamp includes a frame member a having a linear member b such as a wire extended thereon and a clamp region of the original sheet is subjected to exposure from above the sheet clamp such that the original sheet is prevented from deviating from the circumferential surface of the rotary drum. However, in this automatic sheet winding device, in order to prevent the wire b from being cut off by a sheet retaining claw c provided on the rotary drum as shown in FIG. 2, it is required to be so arranged that, for example, a cam d is provided on a side face of the drum so as to space the sheet clamp away from the circumferential face of the drum when the claw c passes by the sheet clamp. Hence, this automatic sheet winding device has such a drawback that when the sheet clamp has been spaced away from the circumferential face of the rotary drum, a portion of the original sheet adjacent to the sheet retaining claw c is relieved of a pressing force and thus, deviates from the circumferential face of the rotary drum, thereby resulting in improper focusing of images of the original sheet at the time of exposure of the original sheet.
Furthermore, in a prior art ADF, a sheet clamping device for clamping an original sheet wound around a rotary drum is provided in an automatic sheet winding device and is mounted on a fixed member so as to prevent the original sheet wound around the rotary drum from deviating from the rotary drum. However, as shown in FIG. 3, the prior art sheet clamping device has been disadvantageous in that since a clamp member 15 for clamping the original sheet is fixedly mounted on a fixed member 16 such as a frame by machine screws, it is difficult to position the clamp member 15 relative to a rotary drum 17 and it is troublesome to remove the clamp member 15 from the fixed member 16.
Moreover, in a further prior art ADF, a claw for gripping an original sheet is provided in an axial direction of a rotary drum so as to grip a leading edge of the original sheet fed by rollers. Thus, the prior art ADF has such a problem that when the original sheet is fed to the claw obliquely, the claw fails to grip the original sheet. In order to solve such a problem of the prior art ADF, various methods for compensating for oblique feed of the original sheet have been proposed. However, even if these methods are employed, such a case happens that the original sheet is fed obliquely again after compensation of oblique feed of the original sheet. In this case, improper winding of the original sheet around the rotary drum cannot be eliminated completely.
In addition, another known ADF includes a detecting device for detecting a transport state of an original sheet through utilization of detection signals from a plurality of sheet sensors provided on a transport path of the original sheet. For example, in a known copying apparatus, etc., a sheet transport mechanism is employed in which the original sheet placed on the transport path is transported. In the known sheet transport mechanism, a detecting means for detecting presence and absence of the transported original sheet is provided but is not capable of properly detecting the transport state of the original sheet.